Polyethylene terephthalate (PET) polyester is commonly prepared by the reaction of ethylene glycol with a terephthalate ester such as dimethyl terephthalate, or less often, with terephthalic acid. This polyester has found wide acceptance in the packaging of food products, in the form of films, sheets and containers.
Blow molding is used to make hollow shaped thermoplastic articles such as containers or bottles for carbonated beverages and the like. In extrusion blow molding, a parison is vertically extruded and thereafter inflated to cause the walls of the parison to assume the shape of a mold. In injection blow molding, a thermoplastic resin is injection molded around a core pin to form a preform and then transferred to a blow mold.
Extrusion blow molding in particular, requires a resin with high melt strength, typically demonstrated by hang time, and with viscoelasticity. However, conventional PET has poor melt strength and tends to drool during extrusion blow molding. Accordingly, there is large commercial demand for a high melt strength PET.
As illustrated by U.S. Pat. No. 4,161,579 to Edelman et al and U.S. Pat. No. 4,609,721 to Kirshenbaum et al, a high melt strength PET polymer modified by the incorporation of a polyfunctional alcohol or acid, chain branching agent is known. The PET polymer may contain a minor amount of diacid co-monomers such as isophthalic acid, adipic acid, 2,6-naphthalenedicarboxylic acid and p-hydroxybenzoic acid. Exemplary chain branching agents include glycerol, sorbitol, hexane triol-1,2,6, pentaerythritol, trimethylolethane, trimethylolpropane, trimethylol benzene-1,3,5, trimesic acid and trimellitic acid.
As exemplified by U.S. Pat. Nos. 4,069,278, 4,132,707 and 4,147,738 to Borman, a branching component containing at least three ester-forming groups, may be incorporated into or blended with a polyalkylene terephthalate prior to processing steps for providing high melt viscosity, thermoplastic block co-polyesters. Illustrative branching components include tri- and tetracarboxylic acids and lower alkyl esters thereof, polyols, dihydroxycarboxylic acids, and hydroxydicarboxylic acids and derivatives such as dimethyl hydroxyterephthalate.
U.S. Pat. No. 4,554,328 to Sinker et al describes a PET polymer suitable for extrusion blow molding. Particularly described is a PET polymer made from 96 parts by weight dimethylterephthalate, 4 parts by weight dimethylisophthalate, and 0.3 parts by weight pentaenythritol, in the presence of an end capping agent. U.S. Pat. No. 4,340,721 to Bonnebat et al describes a PET polyester including 1.5 to 7.5 mol % of recurring units of a crystallization retardant which may be a polyhydric alcohol or a polybasic carboxylic acid such as isophthalic acid or naphthalenedicarboxylic acid, for reducing the residual acetaldehyde concentration. Despite prior work directed to polyethylene terephthalates having high melt strength and reduced acetaldehyde content, the search continues for an improved PET particularly having enhanced melt strength.
There is also a strong commercial demand for a PET resin useful for producing containers or bottles that can be hot-filled between about 85.degree. and 120.degree. C. Normal PET containers can be filled at approximately 80.degree.C. At higher temperatures, these containers usually shrink and/or distort. Advantageously, polyethylene 2,6-naphthalene dicarboxylate (PEN) polyester may be used for such an application due to its higher glass transition temperature. However, PEN is a higher cost material than PET.
To meet the market need for hot fill food and beverage containers at a lower cost, blends of PET and PEN have been produced. U.S. Pat. No. 5,006,613 to Shepherd et al exemplifies a blend preferably containing about 35 to 60 wt. % PET, about 30 to 60 wt. % PEN, and a compatibilizing copolyester for providing clarity. The PET and PEN polyesters may each include up to 15 mol % of a modifying dicarboxylic acid such as isophthalic acid, adipic acid, glutaric acid and so forth.
Containers or bottles formed from a PET/PEN blend or from a copolymer of ethylene terephthalate and ethylene 2,6-naphthalene dicarboxylate, are superior to PET in hot fill applications due to an increased glass transition temperature, and as a result, container shrinkage and distortion are reduced. However, a container characterized by undesired yellowness is aesthetically unpleasing and continues to be a problem associated with processing certain polyesters into bottles or containers.
Therefore, there continues to be a need for a melt strength-enhanced PET having improved hang time and that is useful for blow molding, in particular for extrusion blow molding of bottles or containers. Moreover, there is a continued need for a PET resin useful for hot fill applications.